Conventionally, a voltage induced in the permanent magnet brushless motor is determined by a constant magnetic flux generated by the permanent magnet arranged in a rotor and an angular velocity of an electric motor. In other words, the induced voltage increases in proportion to a speed of the brushless motor. However, when the induced voltage exceeds an inverter's supply voltage, the speed of the brushless motor cannot be increased any more.
Advancing a phase of a current supplied to the brushless motor relative to that of the induced voltage allows more currents to be supplied even with a high induced voltage so that the speed of the brushless motor can be increased.
Such control is called field weakening and has been conventionally known as a technique to extend a high speed operating region. However, advancing the phase of the current relative to that of the induced voltage, in other words, advancing the phase angle of the current, changes a phase relationship between the current and a magnetic flux in air gaps, which increases torque ripple and causes a problem of vibration and noise.
In one conventional technique to reduce the torque ripple in an interior permanent magnet rotor having permanent magnets embedded inside a generally cylindrical rotor, a plurality of slits are formed in the rotor core such as to extend from vicinity of the radially outer side of the magnet of each pole to vicinity of the surface of the rotor.
Further, the plurality of slits are arranged substantially parallel to a radial direction of a pole center and at substantially equal intervals, to reduce the torque ripple, and in turn, to reduce noise and vibration (see, for example, Patent Literature 1).
On the contrary, another method has been proposed, wherein a plurality of slits are formed such as to direct magnetic fluxes emanating from the magnets to converge outside the rotor core laminations, and the converging direction is differed for each of the rotor core laminations, to compensate for a decrease in the rotor characteristics caused by magnetic saturation, and to reduce noise and vibration caused by the torque ripple (see, for example, Patent Literature 2).
In another method that has been proposed, an outer thin portion formed between a radial outer end of slits and the outer circumference of the rotor core is made gradually larger from a pole center toward interpolar portions, so as to reduce a high frequency component in the waveform of the magnetic flux density of the interpolar portions, and to reduce the high frequency component of the induced voltage and cogging torque (see, for example, Patent Literature 3).
In another method that has been proposed to reduce noise and vibration, a plurality of slits are arranged at substantially equal intervals at the radial outer end, wherein, while the interval between the plurality of slits is made larger at the radial inner end in a center portion of the permanent magnet, the interval is made smaller from the center portion toward the ends, to reduce counteracting magnetic flux of an armature, and to improve magnetic flux distribution in the outer circumference of the core (see, for example, Patent Literature 4).
However, while the conventional techniques in these documents can reduce noise and vibration to some extent in normal operation, i.e., without field weakening, these techniques cannot effectively reduce noise and vibration during field weakening with phase advance.
Conventionally, the torque ripple may exceed about 100% when the current phase angle is largely advanced at a highest speed load point during field weakening operation. This is an about five to six times increase in torque ripple as compared to that during low speed operation without any large phase advance.
The present invention has been made to solve this problem and provides a brushless motor capable of high speed rotation by field weakening, which outputs large torque with low noise and low vibration while maintaining mechanical strength.
PTL 1: Unexamined Japanese Patent Publication No. 11-187597
PTL 2: Unexamined Japanese Patent Publication No. 2006-14450
PTL 3: Unexamined Japanese Patent Publication No. 2008-167583
PTL 4: Japanese Patent No. 4248984